I had flying BH with red ENG (but flying). Recently I've hit some tree, the engine immediatelly stopped but I was able to safely land. However even when I have replaced every part in the heli, the engine just won't start when pressing Q. Anyone have any idea?

With just a basic skill I am able to repair every heli to some extent. All glass to orange (it holds fuel) and rear rotor to orange. Engine, main rotor, avionics still red, although I install them in yellow state (almost green). I had problems with almost every heli that it just won't start the engine.

So you said it holds the fuel - seems that the engine is broken to 100% red. Try to install so many engines, until you see an orange condition when you get into the pilots seat, don´t trust the repair menu...

Yes, I have come across this too, seems you need to be proficient in all areas in order to install a good engine.

I installed a body in an offroad mg until i saw the orange hull. I repaired it with 10 scrap metal as far as i know...

I can confirm that a person with at least 2.1 Mechanic skill was able to install engine to 9% (he was able to see repair percentage) and the heli was able to start... so yes, it was a skill problem.